Low-end displays typically require only monochromatic LEDs. Applications include simple sporting scoreboards, single-line scrolling displays, and transportation road signs. A newer and growing market for high-quality video displays requires the capability to play full-motion video shown in millions of colors. These applications include ever-expanding advertising markets encompassing convenience stores, retail shops, gas stations, and stadiums.
An emerging market for LEDs is in DLP-based televisions and LCD-based televisions. Accurate color reproduction by these televisions is dependent on the available colors of the televisions' backlight. Proper control of red LEDs, green LEDs, blue LEDs (collectively referred to as RGB LEDs), and white LEDs (“WLEDs”) produce a color spectrum that is larger than the NTSC color spectrum for television broadcasts. By contrast, the backlighting of a cold cathode fluorescent lamp (“CCFL”) produces about 85% of the NTSC color spectrum.
Thus, sophisticated LED drivers capable of providing multiple brightness levels are required. The number of colors available in the display is proportional to the number of brightness levels available for each of the RGB LEDs that make up a single pixel in an overall display. Competition between display manufacturers is driving designers toward high-end LED drivers with integrated PWM functionality capable of delivering thousands of brightness levels. An increased number of brightness levels can enhance color shading and improve video quality.
High-quality, full-color video requires hundreds or thousands of brightness levels between 0% and 100%. Older LED drivers use analog dimming circuits to provide these brightness levels. Analog dimming circuits alter the brightness of a LED display by adjusting the forward electric current of the LEDs. For example, if an LED is at full brightness with 20 mA of forward current, then 25% brightness is achieved by driving the LED with 5 mA of forward current. While this dimming scheme is simple and works well for lower-end displays, a substantial drawback with analog dimming is that an LED's color shifts with changes in forward current.
Pulse width modulation (“PWM”) dimming can be used to adjust LED brightness levels while maintaining superior color quality. This technique is also referred to as PWM gray scaling. PWM dimming is achieved by applying a maximum forward current for maximum brightness at a reduced duty cycle. In other words, the LED's brightness is controlled by adjusting the relative ratios of an amount of time that an LED is on to an amount of time that the LED is off. A 25% brightness level is achieved by turning the LED on at maximum forward current for 25% of each duty cycle period. To avoid display flicker, the switching speed must be greater than 60 Hz. Above 60 Hz, the human eye averages the LED's on-time and the LED's off-time, seeing only an effective brightness that is proportional to the LED's on-time duty cycle. An advantage of PWM dimming is that the forward current is always constant. Therefore, the LED's color does not vary since the brightness level does not vary, as is the case with analog dimming schemes. Furthermore, precise brightness levels can be achieved while preserving the color purity by switching the LED off and on.
Since this type of PWM dimming is microprocessor-driven, it is limited to a maximum number of discrete brightness levels for each LED, commonly referred to as grayscale steps. The total available number of discrete steps during any one period determines the LED's brightness resolution. High-quality displays require hundreds to thousands of brightness levels to accurately reproduce the full color spectrum necessary for full-motion video.
Unfortunately, PWM dimming controls have other problems for displaying images. For instance, FIG. 1a illustrates a prior art circuit for driving a plurality of diodes in parallel using a PWM dimming control. Due to non-ideal variations for each diode, the I-V characteristics of each diode may vary. Therefore, the current across each diode (I1, I2, I3, and I4) may be different. Thus the brightness level of each diode may not be matched to each other (i.e., the brightness levels of the diodes are not the same). This can lead to uneven brightness on the respective display that houses the diodes.
FIG. 1b illustrates another prior art circuit for driving a plurality of diodes in series using a PWM dimming control circuit. The brightness levels of the diodes will be similar since the current, I5, is the same over each diode. Thus the brightness level of a diode will be matched with the brightness levels of the other diodes. However, this implementation is costly and does not allow for multiple channels of diodes.
FIG. 1c illustrates a prior art circuit for driving a plurality of diodes using a PWM dimming controller, where a voltage feed back loop is connected to the PWM dimming controller and a single channel. The voltage feed back loop is used to match the current over one channel. However, the implementation of this display may lead to uneven brightness levels for the other channels since the diodes in other channels are not fed back to the PWM dimming control block.
FIG. 1d illustrates a prior art circuit for driving a plurality of diodes using a PWM dimming controller, where a current balancer module is used to select a feed back voltage from a plurality of channels. A current balancer is used to detect the various voltages on each channel, and feed back these voltages to a PWM dimming control to adjust the current over each channel. A drawback of this design is that the current balancer and the PWM dimming controller are two separate circuits; thus leading to time lag, increased implementation complexity, and inefficient power consumption. Furthermore, existing prior art methods for the current balancer have a very large error rate of 10 percent or more.
FIG. 1e illustrates a prior art circuit for driving a plurality of diodes using a PWM dimming control, where a PWM dimming control and a current balancer are integrated in one block. Although this design is more advantageous in certain respects than the previous example, implementation complexity is still not optimized and the current balancing integrated module can have a relatively high error rate of 10 percent or more.
Therefore, it is desirable to provide methods for PWM dimming controls for LEDS, where respective currents for each channel of diodes are matched to maintain even display brightness at a very low error rate.
Another problem with PWM dimming controls is that the currents over the LED channels are not matched due to variations of the resistors of the current source. FIG. 1f illustrates a prior art method for current sources for driving a plurality of LED channels. In a current source 1, a channel VCH1 is connected to an operational amplifier via a transistor. A voltage applied on the negative input of the operational amplifier can be denoted VFB1. A voltage applied at the source drain of the transistor 802 can be denoted VCS1. This nomenclature can be extended to a current source 2 (e.g. VFB2 and VCS2), and so forth for the other current sources since schematically all the current sources are substantially similar. Each current source drives a single channel of LEDs.
There can be at least three resistances for each current source (e.g. in the current source 1, a resistance due to an operational amplifier, RCSREF, and RCS1). These resistances must be matched with other current sources to generate an accurate current through the channel. However, prior art methods do not effectively match these three resistances, thus the currents in the plurality of channels may not be accurately matched to one another. Therefore, methods for improved current matching over a plurality of channels are required. This is especially true when the channels comprise of LEDs since current matching is of particular importance to maintain the same brightness level for the LEDs of different channels.
FIG. 1g illustrates another prior art method for channel matching, where only one RCSEF is used for a plurality of current sources. Here, for each current source only an operational amplifier and a resistor (e.g. RCS1) need to be matched. Current mismatch comes from mismatch between {RCS1, RCS2}, {VCSREF1, VFB1}. However, this also results in inaccurate currents because RCS1, RCS2 . . . RCS8 are generally not physically laid out close together. Therefore, methods for channel matching are required that can produce accurately matched currents over the plurality of channels.
Yet another problem related to PWM dimming controls circuits is generating a compensating ramp signal for a power converter circuit. Conventional current-mode controlled DC to DC converters operating above 50% duty cycle need a compensating ramp signal superimposed on a current sense signal, which is used as a control parameter, to avoid open loop instability and sub-harmonic oscillation problems.
In a typical voltage mode boost converter, an inductor is placed between a power supply and the drain of a switching transistor. A diode is coupled between the common inductor/drain terminal and the output of the converter. As the switching transistor is turned on and off under the control of a pulse width modulator, the inductor is energized with a current which flows through the inductor and switching transistor to ground, thus storing energy in the core of the inductor in the form of a magnetic flux. When the switching transistor is turned off, current continues to flow through the inductor. As the magnetic flux field collapses, a voltage appears across the inductor which is delivered through a diode to the load. Typically, a large capacitor is placed across the output of the converter to hold the converter output voltage at a predetermined level during the periods when the switching transistor is charging the inductor.
In a voltage controlled converter, the voltage appearing at the load is sensed by an error amplifier. The error amplifier generates an error voltage which is related to the voltage appearing at the output of the converter. Minute changes in voltage appearing at the output of the converter are changed to relatively larger voltage swings by the error amplifier. The output of the error amplifier is coupled to one terminal of a comparator which has another terminal coupled to a compensating ramp signal. A current sensing circuit may generate the ramp signal as a function of the current from the inductor. As the voltage appearing at the output of the error amplifier rises and falls with respect to the compensating ramp signal, the output of the comparator changes state in a pulse-width modulated waveform. This signal is coupled to the switching transistor to effect the switching thereof and complete the regulator loop.
However, it remains an ongoing goal to generate a compensating ramp signal to optimize the stability of the power converting circuit. In addition, it remains an ongoing goal to increase the accuracy for sensing the inductor current since switches such as MOSFETS have process variation (e.g., based on temperature) which can cause inaccuracies during voltage measurements.